Fix cumsum error for tensors with zero elements (#31694)
Summary:
Currently `cumsum` crashes for tensors with non-empty dimensions but with zero elements, which could happen when some dimension is zero. This commit fixes the error by checking both `dim()` and `numel()` in cumsum backward
Fixes https://github.com/pytorch/pytorch/issues/31515
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31694
Reviewed By: mrshenli
Differential Revision: D19266613
Pulled By: leedtan
fbshipit-source-id: 9407e0aa55440fed911c01a3580bb6c5eab62a16
